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41

Conducting Nylon fibers were prepared by in situ polymerization of

aniline on to the fiber surface, after providing a chemical etching

treatment to the fibers using chromic acid. The properties of the etched

and polyaniline coated fibers were evaluated using scanning electron

microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy,

X-ray diffraction, thermogravimetry and differential scanning

calorimetry. Though the etching process caused a marginal decline in the

mechanical properties of the fiber, it provided a reasonably rough surface

for PANI adhesion and enhanced the conductivity of the fiber. The

conductivity increased from 4.22 × 10-2

S/cm to 3.72 × 10-1 S/cm at an etching

time of 4 h.

Chapter 2

Polyaniline coated short Nylon fiber∗∗∗∗

2.1 Introduction

Conductive polyaniline (PANI)/polymer fibers can be produced either by inclusion

of PANI or by in situ oxidative polymerization of aniline monomer. Owing to low

thermal stability and insolubility of PANI, the former method is hardly acceptable for

producing conductive composite fabrics. Therefore, most studies have focused on in

situ polymerization route to produce conductive fabrics. This method does not

require destruction of the substrate and provides reasonably good conductivity and

∗ A part of the work described in this chapter has been presented at:

� International Conference POLYCHAR-16, World Forum of Advanced

Materials, Feb 17-21, 2008, World Unity Convention Center, Lucknow, India

� 17th

AGM of Materials Research Society of India, AGM-MRSI, Feb 13-15,

2006, University of Lucknow, India

� National Conference on Frontiers in Polymer Science and Technology,

POLYMER 2006, Feb 10-12, 2006, Indian Association for the Cultivation of

Science, Kolkata, India

Chapter 2

42

produces modified polymer matrices with a PANI layer at their surface. The

thickness and conductivity of the layers depend on the method of modification and

on the time of contact of the solid matrix with the reaction medium. The surface

conductivity obtained may range, from semiconductor level up to the conductivity of

pure PANI. Even a simple dipping method resulted in conductivity of 1-5 S/cm and

transmittance of 80 % at 450-650 nm for a 0.5 µm PANI layer [1]. Apparently, this

method is not technically suitable for sheet materials, because it requires the use of

polymer matrixes with a good adhesion to PANI. Moreover, it produces pure PANI

deposits having poor mechanical properties. At the same time, for fibers and textile

materials with a well-developed reactive surface, it may lead to the production of

conducting fibers and fabrics with PANI grafted on the surface and inside the pores.

Genies et al. have reported the impregnation of PANI onto glass textiles [2].

Chemical polymerization of polypyrrole on poly(ethylene terephthalate) fabric was

reported by Kim et al. [3]. Aniline can be polymerised on the fabrics from the

aqueous solution [4-6] or the vapor phase [7] using appropriate oxidizing agents. The

use of peroxosalts as oxidants causes a graft copolymerization of aniline and its

derivatives onto a polymer matrix [8]. Anbarasan and co-workers investigated the

kinetics of this grafting onto rayon [9], wool [10] and PET [11] fibers and proposed a

possible mechanism of graft and homo polymerization of aniline. Specifically, they

carried out oxidative chemical polymerization of aniline using peroxydisulfate and

peroxomonosulfate as the sole initiator in an aqueous acidic medium in the presence

of the fibers. This resulted in the chemical grafting of PANI onto the fibers,

confirmed by Fourier Transform Infrared spectroscopy (FTIR), cyclic voltammetry,

weight loss study, and conductivity measurements. The authors proposed a probable

mechanism involving graft polymerization of aniline through interaction of the

oxidant with the fiber surface, inducing the formation of radical sites at the fiber

surface, followed by grafting aniline with its subsequent participation in a typical

aniline oxidative polymerization. The conductivity of such fabrics depended on

deposition of conductive polymer on the surface or in the interstices of the fabric [4, 12].

A PANI layer bonded strongly to the fiber surface is required for better conductivity

PANI coated short Nylon fibers

43

and durability. This can be achieved by improving the adhesion of PANI on the fiber

surface.

Two methods to obtain electrically conductive fabrics by in situ polymerization of

aniline were compared by Oh and coworkers [13]. These materials were prepared by

immersing the Nylon fabrics in pure aniline or an aqueous hydrochloride solution of

aniline followed by initiating the successive direct polymerization in a separate bath

(DPSB) or in a mixed bath (DPMB) of oxidant and dopant solution with aniline.

They showed that the DPMB process produced higher conductivity in the composite

fabrics, reaching 0.6 × 10-1

S/cm. Moreover, this process induced a smaller decrease

in the degree of crystallinity than the DPSB process. The PANI/Nylon composite

fabrics displayed a good serviceability. Thus, no important changes in the

conductivity were observed after abrasion of the composite fabrics over 50 cycles

and multiple acid and alkali treatment. The stability of the conductivity decreased by

less than one order after exposure to light for 100 h, but it was significantly

decreased after washing with a detergent.

In another work, the same authors attempted plasma treatment of the fabrics to

improve the surface adhesion between PANI and Nylon fabric and hence the

serviceability, conductivity and durability [14]. But this is an expensive method. An

alternate, less expensive method is chemical etching of the fiber surface. The

monomer and the oxidant are adsorbed on the surface of the fiber instead of diffusing

into the fiber. Therefore, it is expected that the fabric conductivity and adhesion of

the polymer on the fiber surface can be improved with an increase in the surface

energy and surface area. This can be achieved by the chemical etching process. In

this work, we have explored the possibility of using chromic acid for improving the

adhesion properties and conductivity of Nylon fibers. Optimization of chromic acid

concentration and etching time and the electrical properties of the PANI deposited

etched Nylon fiber are presented. Surface characteristics of the etched fibers were

analyzed using scanning electron microscopy, X-ray photoelectron spectroscopy,

infrared spectroscopy and X-ray diffraction analysis. The thermal characterization of

the coated fibers is also presented.

Chapter 2

44

2.2 Experimental

2.2.1 Materials

Nylon-6 fibers (1680/2D) were obtained from Sri Ram Fibers Ltd., Chennai, India.

Aniline, ammonium peroxydisulfate, hydrochloric acid and chromium trioxide were

supplied by S. D. Fine Chemicals Ltd., Mumbai, India.

2.2.2 Preparation of polyaniline

An aqueous solution of ammonium peroxydisulfate (1.62 M) was added drop wise to

a solution of 0.0219 M aniline in 50 ml, 1 N hydrochloric acid at ambient

temperature. After complete addition, the reaction was left to proceed for 4 h. The

precipitate formed was then filtered, washed with acetone till the filtrate became

colorless, followed by a wash with dilute hydrochloric acid and finally with water.

The PANI formed was then dried in an air oven at 60 °C.

2.2.3 Etching treatment

1 g of the chopped Nylon fibers (5 mm) were immersed in 250 ml of aqueous

chromic acid solution of a specified concentration at 60-65 °C with continuous

stirring for a specified time. The etched fibers were subsequently filtered and washed

with distilled water till the washings were free from coloration and then dried at

60 °C in an air oven.

2.2.4 In situ polymerization

0.5 g of the etched fiber was soaked in 10 ml of aniline for 24 h at room temperature.

The excess aniline was then drained off and the fibers were blotted with tissue paper.

The weight of the fiber before and after soaking was noted. The percentage of aniline

absorption was determined to be 23.8 %. The soaked fibers were then added to a

solution of 0.0219 moles of aniline in 50 ml, 1 N hydrochloric acid. The

polymerization was carried out for 4 h at room temperature using an aqueous

solution of ammonium peroxydisulfate (1.62 M). These were then filtered, washed

with water till the washings became colorless and further dried.


45

2.2.5 Characterization

2.2.5.1 Tensile strength

The mechanical properties of the fibers were studied using a Shimadzu Universal

Testing Machine (UTM model AG I) with a load cell capacity of 10 kN. The gauge

length between the grips at the start of each test was adjusted to 50 mm. The fibers of

254 mm length were held between the two grips and a uniform rate of grip separation

(cross-head speed) of 20 mm/min was applied. The strength was evaluated after each

measurement automatically by the microprocessor and presented on a visual display.

Average of atleast six sample measurements were taken to represent each data point.

2.2.5.2 Scanning electron microscopy (SEM)

Scanning electron microscopy is a very useful tool to gather information about

topography, morphology, composition and micro structural information of materials.

In a typical SEM, electrons are thermionically emitted from a tungsten or lanthanum

hexaboride (LaB6) cathode and are accelerated towards an anode; alternatively,

electrons can be emitted via field emission. The electron beam, which typically has

an energy ranging from a few hundred eV to 100 keV, is focused by one or two

condenser lenses into a beam. Characteristic X-rays are emitted when the primary

beam causes the ejection of inner shell electrons from the sample and are used to tell

the elemental composition of the sample. The back-scattered electrons emitted from

the sample may be used alone to form an image or in conjunction with the

characteristic X-rays. These signals are monitored by detectors (photo multiplier

tubes) and magnified. An image of the investigated microscopic region of the

specimen is thus observed in cathode ray tube and is photographed.

The SEM images of the short fibers were obtained using a Cambridge Instruments

S 360 stereo scanner (version V02-01). The fibers were mounted on a metallic stub

and an ultrathin coating of electrically conducting material (gold) is deposited by low

vacuum sputter coating. This is done to prevent the accumulation of static electric

fields at the specimen due to the electron irradiation during imaging. Another reason

Chapter 2

46

for coating, is to improve contrast. The SEM is capable of producing high-resolution

images of a sample surface.

2.2.5.3 X- ray photoelectron spectroscopy (XPS)

For each and every element, there will be a characteristic binding energy associated

with each core atomic orbital i.e. each element will give rise to a characteristic set of

peaks in the photoelectron spectrum at kinetic energies determined by the photon

energy and the respective binding energies. The presence of peaks at particular

energies therefore indicates the presence of a specific element in the sample under

study. Furthermore, the intensity of the peaks is related to the concentration of the

element within the sampled region. The shape of each peak and the binding energy

can be slightly altered by the chemical state of the emitting atom. Hence XPS can

provide chemical bonding information as well. XPS is not sensitive to hydrogen or

helium, but can detect all other elements. Thus, X-ray photoelectron spectroscopy is

a quantitative spectroscopic technique that measures the elemental composition,

empirical formula, chemical state and electronic state of the elements that exist

within a material. XPS is also known as ESCA, an abbreviation for Electron

Spectroscopy for Chemical Analysis.

The phenomenon is based on the photoelectric effect. XPS spectra are obtained by

irradiating a material with a beam of aluminum or magnesium X-rays while

simultaneously measuring the kinetic energy and number of electrons that escape

from the top 1 to 10 nm of the material being analyzed. XPS requires ultra-high

vacuum conditions. Commercial XPS instruments use either a highly focused 20 to

200 µm beam of monochromatic aluminum K-alpha X-rays or a broad 10-30 mm

beam of non-monochromatic Mg X-rays.

The surface of the fibers was analyzed using an X-ray induced photoelectron

VG Scientific ESCA-3000 spectrometer using a non-monochromatic Mg Kα

radiation (1253.6 eV). First a general scan was recorded from 0-1000 eV at steps of

1 eV to confirm the presence of C, N and O in the sample. Then, narrow scans were

done for C1s, N1s and O1s at steps of 0.1 eV. To correct for possible deviations caused


47

by electric charge of the samples, the C1s line at 285 eV was taken as internal

standard.

2.2.5.4 Infrared spectroscopy (IR)

Infrared spectroscopy is the absorption measurement of different IR frequencies

(400-4000 cm-1

) by a sample positioned in the path of an IR beam. Infrared

spectroscopy exploits the fact that molecules have specific frequencies at which they

rotate or vibrate corresponding to discrete energy levels. These resonant frequencies

are determined by the shape of the molecular potential energy surfaces, the masses of

the atoms and, by the associated vibronic coupling. In order for a vibrational mode in

a molecule to be IR active, it must be associated with changes in the permanent

dipole. The frequency of the vibrations can be associated with a particular bond type.

The infrared spectrum of a sample is collected by passing a beam of infrared light

through the sample. Examination of the transmitted light reveals how much energy

was absorbed at each wavelength. This can be done with a monochromatic beam,

which changes in wavelength over time, or by using a Fourier Transform instrument

to measure all wavelengths at once. From this, a transmittance or absorbance

spectrum can be produced, showing at which IR wavelengths the sample absorbs.

Analysis of these absorption characteristics reveals details about the molecular

structure of the sample. This technique works almost exclusively on samples with

covalent bonds.

Infrared spectra of the fibers were recorded on a Bruker FTIR spectrophotometer

model Tensor 27 (spectral range of 7500 cm-1

to 370 cm-1

with standard KBr

beamsplitter) in attenuated total reflectance (ATR) mode. It uses zinc selenide as the

crystal material with high sensitivity DLATGS detector with KBr window.

2.2.5.5 X- ray diffraction (XRD)

X-rays are electromagnetic radiation of wavelength about 1 A°, which is about the

same size as an atom. X-ray diffraction has been in use in two main areas, for the

fingerprint characterization of crystalline materials and the determination of their

Chapter 2

48

structure. Each crystalline solid has its unique characteristic X-ray powder pattern

which may be used as a "fingerprint" for its identification. Once the material has

been identified, X-ray crystallography may be used to determine its structure, i.e.

how the atoms pack together in the crystalline state and what the interatomic distance

and angle are, etc. X-ray diffraction is one of the most powerful characterization

tools used in solid state chemistry and materials science. We can determine the size

and the shape of the unit cell for any compound most easily using the diffraction of

X-rays.

X-ray diffractograms of the fibers were recorded using a Bruker AXS D8 Advance

Diffractometer using CuKα radiation (λ = 1.54 A°) at 35 kV and 25 mA with a

smallest addressable increment of 0.001 °. XRD results were obtained in the range

2θ = 3 ° to 80

° at a scan rate of 4

°/min.

2.2.5.6 Thermogravimetric analysis (TGA)

Thermogravimetric analysis is a type of testing that is performed on samples to

determine changes in weight in relation to change in temperature. Such analysis

relies on a high degree of precision in three measurements: weight, temperature, and

temperature change. TGA is commonly employed in research and testing to

determine characteristics of polymers, to determine degradation temperatures,

absorbed moisture content of materials, the level of inorganic and organic

components in materials, decomposition points of explosives, solvent residues and

composition of blends and composites. The analyzer usually consists of a high-

precision balance with a pan loaded with the sample. The sample is placed in a small

electrically heated oven with a thermocouple to accurately measure the temperature.

The atmosphere may be purged with an inert gas to prevent oxidation or other

undesired reactions. A computer is used to control the instrument. Analysis is carried

out by raising the temperature gradually and plotting weight against temperature.

Thermogravimetric studies lend a hand in determining the thermal stability of the

fibers. The TGA instrument produces a continuous record of weight as a function of

temperature. TGA studies were carried out on a Q-50, TA Instruments


49

thermogravimetric analyzer (TGA) with a programmed heating of 20 °C/min from

ambient to 800 °C. The chamber was continuously swept with nitrogen at a rate of

90 ml/min. The onset temperature of degradation was recorded and temperature at

maximum degradation was taken as the peak degradation temperature. The weight

percentage of the samples remaining at 800 °C was recorded as the residue.

2.2.5.7 Differential scanning calorimetry (DSC)

Differential scanning calorimetry is a technique for measuring the energy necessary

to establish a nearly zero temperature difference between a substance and an inert

reference material, as the two specimens are subjected to identical temperature

regimes in an environment heated or cooled at a controlled rate. Both the sample and

reference are maintained at nearly the same temperature throughout the experiment.

The basic principle underlying this technique is that, when the sample undergoes a

physical transformation such as phase transitions, more (or less) heat will need to

flow to it than the reference to maintain both at the same temperature. Whether more

or less heat must flow to the sample depends on whether the process is exothermic or

endothermic. By observing the difference in heat flow between the sample and

reference, differential scanning calorimeters are able to measure the amount of heat

absorbed or released during such transitions.

Differential scanning calorimetry was employed to determine the degree of

crystallinity, heat of fusion and melting temperature of the fibers. The measurements

were conducted using a DSC Q-100, TA Instruments calorimeter having a

temperature accuracy of ± 0.1 °C. Analyses were done in nitrogen atmosphere using

standard aluminum pans. The samples were exposed to a heating rate of 10 °C/min

from ambient to 300 °C. The peak temperature of the melting endotherm was taken as

the melting temperature; Tm, and the heat of fusion; ∆Hƒ, was determined from the

area of the melting endotherm. The degree of crystallinity; χc, was determined by

normalizing the observed heat of fusion of the sample to that of a 100 % crystalline

sample of Nylon, which is available from literature [15, 16].

Chapter 2

50

2.2.5.8 DC electrical conductivity

The DC electrical conductivity of the fibers was measured by a two-probe method

using a Keithley 2400 source-measure unit, which is a fully programmable

instrument capable of sourcing and measuring voltage or current simultaneously with

accuracy. A constant current source was used to pass a steady current through one of

the probes and the voltage across the other was measured. The measurements were

done at room temperature. The conductivity of the sample was calculated by the

following formula:

)1.2(

where, σ is the electrical conductivity, I is the current through the probe in amperes,

V is the voltage across the probe in volts, l is the spacing between the probes in

centimeters and A is the area of contact of the probes with the sample in centimeter

square.

2.3 Results and discussion

2.3.1 Characterization of polyaniline

2.3.1.1 Scanning electron microscopy

The SEM micrograph of pristine PANI can be seen in fig. 2.1. It shows aggregated

granular form and lacks macroscopic molecular orientation. Several reports on such a

granular morphology of PANI are available in the literature [17]. Especially, such

morphology is obtained in the absence of a surfactant [18]. The shape of the

chemically polymerized polyaniline particles is generally difficult to be controlled in

a bulk polymerization process. PANI aggregates are made of small grains comprised

of a collection of small spheres, which in turn, is comprised of smaller spheres built

from still smaller primary particles. These primary particles have a metallic core

surrounded by an amorphous non-metallic shell. Haba and coworkers calculated the

average size of the DBSA doped primary PANI particles to be 18.7 nm using Small

angle X-ray scattering (SAXS) [19]. These primary particles generate aggregates,

)/()/()/( AlVIcmS ×=σ


51

which further cluster into agglomerates. These agglomerates, about 50 µm in size, are

located within gel-like units, which are structured due to the formation of hydrogen

bonds between free DBSA molecules. The granular morphology obtained for PANI

is presumed to be due to the characteristic feature of the hydrochloric acid dopant

used in the preparation. There are reports which state that the dopant size and its

nature affect the surface morphology of the composite. Oh et al. synthesized PANI

with various acidic dopants with an electrochemical method and found that the

dopants had a great influence on the morphology of PANI [20].

Fig. 2.1 Scanning electron micrograph of pristine PANI

2.3.1.2 Infrared spectroscopy

Fig. 2.2 presents the IR spectrum of pristine PANI. The absorption at 3235 cm-1

is

assigned to the NH+ group and indicates the protonated PANI salt. There were no

features in the spectra between 2800 cm-1

and 1650 cm-1

, because no functional

groups of PANI showed vibration absorption peaks in this region. The peaks at 1569 cm-1

and 1492 cm-1

are due to the C=C stretching in the quinoid ring and benzenoid ring,

respectively [21-23]. The peak at 1299 cm-1

is due to the C-N stretching of the

secondary amine of the PANI backbone. A strong band characteristically appears at

1146 cm-1

which has been explained as an electronic band or a vibrational band of

nitrogen quinone, an in-plane bending vibration of imino-1,4-phenylene, and has

been reported to be associated with the electrical conductivity of PANI [22-25].

Chapter 2

52

The -C-H aromatic out of plane bending mode is a key to identify the type of

substituted benzene. For PANI, this mode is observed as a single band at 817 cm-1

,

which falls in the range 800-860 cm-1

and is reported for 1, 4 disubstituted benzene

[26-28].

3300 2750 2200 1650 1100 550

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

817 c

m-1

1146 c

m-1

1299 c

m-1

1492 c

m-1

1569 c

m-1

3235 c

m-1

AT

R u

nit

s

Wavenumber (cm-1)

Fig. 2.2 IR spectrum of pristine PANI

2.3.1.3 X-ray diffraction analysis

The XRD pattern of pristine PANI was obtained as shown in fig. 2.3.

10 20 30 40

0

1

2

3

4

5

6

7

8

9

10

Inte

nsit

y c

ou

nts

Two-theta-scale

Fig. 2.3 XRD spectrum of pristine PANI

Pristine PANI gives diffraction peaks at 2θ ≈ 11 °, 15

°, 17.5

°, 21

°, 26

° and 30

°.

Poughet et al. also have reported similar diffraction peaks for PANI [29]. Ram and


53

coworkers reported sharp peaks at 2θ ≈ 9 °, 20

°, 25

°, 30

° and 35

° [26]. Moon et al.

reported that the peak at 2θ ≈ 10 °

arises from scattering with momentum transfer

approximately parallel to the polyaniline chains [30].

2.3.1.4 Thermal analysis

The TGA trace of pristine PANI is presented in fig. 2.4. PANI shows three

degradations. The first one is due to the loss of moisture, which ends at 139 °C. The

second degradation, which starts at 153 °C, ends at 295

°C and corresponds to a

weight loss of 7.4 % is due to the dopant evolution. The final one in the range

342-689 °C is due to degradation of the PANI chain [31]. About 70 % weight loss

occurs during this degradation. This is the general thermal behavior of PANI [32].

PANI doped with DBSA too exhibited similar degradations [33]. The possibility of

stable carbonaceous char formation is greater in the case of aromatic compounds and

hence PANI leaves a residue of 5.19 %.

0 100 200 300 400 500 600 700 800 900

0

20

40

60

80

100

Residue

5.19 %

16.8 %

7.4 %

70.1 %

Weig

ht

(%)

Temperature (oC)

Fig. 2.4 TGA trace of pristine PANI

The DSC curve of PANI is shown in fig. 2.5. The PANI powder shows two

endotherms; the first one between 100 °C and 190

°C due to deprotonation and the

second one due to the PANI backbone degradation starting at about 200 °C.

Chapter 2

54

50 100 150 200 250

Exo up

125.1 oC

Hea

t fl

ow

(W

/g)

Temperature (oC)

Fig. 2.5 DSC curve of PANI

2.3.2 Etching of Nylon fiber

Chemical etching treatment using chromic acid can be an effective method to

increase aniline deposition and polymerization on Nylon fiber. The etching

conditions were optimized with reference to concentration of chromic acid and time

of etching. Nylon fibers were treated with chromic acid of differing concentrations

and to different time durations.

2.3.2.1 Strength of the fiber

Effect of etching conditions on the strength of the fiber was determined as a function

of acid concentration and time of etching. Fig. 2.6 shows the strength of the fiber

etched with different concentrations of chromic acid for a period of 6 h. There is a

regular decrease in the strength of the fiber on etching. This may be attributed to the

possible degradation of the Nylon fibers. Higher concentrations drastically reduce the

strength of the fiber. Hence the concentration of chromic acid was optimized at

0.012 N for further studies.


55

0.00 0.01 0.02 0.03 0.04 0.05 0.06

0.02

0.03

0.04

0.05

0.06

0.07

Bre

akin

g lo

ad

(kN

)

Chromic acid concentration (N)

Fig. 2.6 Variation of the strength of Nylon fiber with chromic acid concentration

The strengths of the fiber etched to different extents of time are depicted in fig. 2.7.

There is a major drop in the strength of the fiber during the initial one hour, after

which the fall slows down and, stabilizes at 4 h. Beyond this, the strength further

drops. Hence the optimum time of etching is found to be 4 h.

0 1 2 3 4 5 6

0.052

0.054

0.056

0.058

0.060

0.062

0.064

0.066

Bre

akin

g lo

ad

(kN

)

Time of Etching (h)

Fig. 2.7 Variation of strength of Nylon fiber with etching time

2.3.2.2 Surface characteristics

The morphology changes in the Nylon fiber surface on etching with chromic acid

were observed with SEM. Fig. 2.8 shows the scanning electron micrographs of virgin

fiber and fiber etched with 0.012 N chromic acid for 4 h. Clean and smooth surface is

Chapter 2

56

observed for virgin Nylon fiber. The etched fiber shows an irregular surface with

minor cracks, probably arising from the hydrolysis of the fiber surfaces in the acidic

medium. Such irregularity in the fiber surface is expected to help in the physical

anchoring and to improve adhesion of PANI on the fiber.

Fig. 2.8 Scanning electron micrographs of (a) virgin fiber and (b) etched fiber

The general scan in the XPS spectra of the virgin and etched fibers show the

presence of C, N and O. Individual scans were done for C, N and O. The peak in the

XPS spectrum of the virgin fiber is at 406.2 eV and that of the etched fiber is at

399.4 eV (fig. 2.9). This change in the peak position might be due to the difference in

the environment of the N-H bonds. The peak at 406.2 eV in the virgin fiber may be

due to that of N-H bond and the peak at 399.4 eV is due to the H-N-H bond. This

means that as a result of etching, hydrolysis of the amide linkage has taken place

changing the amide NH to primary amine. The intensity of the N-H peak is decreased

in the case of etched fibers as the amide group hydrolyses to yield NH2 groups.

a b


57

396 398 400 402 404 406 408

b

a

406.2

399.4

Inte

ns

ity

Binding Energy (eV)

Fig. 2.9 XPS spectra of (a) virgin fiber and (b) etched fiber

The infrared spectra of the virgin and etched fibers are presented in fig. 2.10.

3300 2750 2200 1650 1100 550

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

1415 c

m -

1

1538 c

m -

1

1634 c

m -

1

2928 c

m-1

3297 c

m -

1

AT

R u

nit

s

Wavenumber (cm-1)

a b

Fig. 2.10 IR spectra of (a) virgin fiber and (b) etched fiber

The absorption at 3297 cm-1

is due to the N-H stretching; peak at 2928 cm-1

is due to

the C-H stretching; peaks at 1634 cm-1

and 1538 cm-1

can be assigned to C=O

stretching in the amide and N-H bending in secondary amide (-N-H), respectively,

and the peak at 1415 cm-1

is assigned to the C-N stretching of amide group in Nylon

[32, 34]. The ratio of intensities of absorption corresponding to N-H and C-H

vibrations (N-H/C-H intensity ratio) of the etched fiber increased to 1.625 from 1.109

of the virgin fiber, indicating that hydrolysis of the amide linkage has taken place

during etching. The N-H bonds change to NH2 during hydrolysis. Since there are

Chapter 2

58

more number of H atoms per N atom in the NH2 bonds, the intensity of the N-H

peaks will be higher. This indicates that the etching process induces hydrolysis of the

amide linkages in the Nylon fiber.

Fig. 2.11 presents the XRD patterns of the virgin and etched fibers. The virgin fiber

shows maxima at 2θ ≈ 20.2 ° and 23.5

°, which are reasonably close to the values

assigned by previous researchers for the α1 and α2 peaks, respectively [13, 35]. The

etched fibers showed maxima at 2θ ≈ 20.3 ° and 23.6

°. Upon etching, the intensity of

the α1 peak reduces slightly due to the slight decrement in the percentage crystallinity

as confirmed by the DSC measurements. Oh et al. have also reported such a

decrement in intensity owing to a decrease in the percentage crystallinity [13].

5 10 15 20 25 30 35 40 45

0

5

10

15

20

25

30

35

40

Inte

nsit

y c

ou

nts

Two-theta-scale

a b

Fig. 2.11 X-ray diffraction patterns of (a) virgin fiber and (b) etched fiber

2.3.2.3 Thermal characteristics

The TGA traces of the virgin and etched fibers are given in fig. 2.12 and the thermal

degradation characteristics are presented in table 2.1. The thermograms show only

one major weight loss attributed to the structural decomposition [25] starting at

~ 340 °C for the virgin fiber and ~ 330

°C for the etched fiber. The onset temperature

of degradation, the peak degradation temperature and the temperature at 50 % weight

loss decrease on etching pointing to the decreased thermal stability of the etched

fiber. But the etched fiber gives a lesser weight loss during the degradation and

higher amount remains at the peak degradation temperature.


59

0 100 200 300 400 500 600 700 800 900

0

20

40

60

80

100

Weig

ht

(%)

Temperature (oC)

a b

Fig. 2.12 TGA traces of (a) virgin fiber and (b) etched fiber

Table 2.1 Thermal decomposition data of the virgin and etched fibers

Virgin

fiber

Etched

fiber

Onset temperature (°C) 342.9 331.9

Peak degradation temperature (°C) 461.1 452.9

Weight loss (%) 94.9 92.4

Weight remaining at Peak degradation temperature (%) 33.4 37.7

Temperature at 50 % weight loss (°C) 453.5 447.1

The melting temperature, heat of fusion and degree of crystallinity of the fibers were

determined by differential scanning calorimetry and are presented in table 2.2.

Fig. 2.13 shows the DSC curves of the virgin and etched fibers. All these parameters

decrease upon etching, as expected. This decrease may be attributed to the

degradation of the fiber surface during the etching process.

Chapter 2

60

180 200 220 240

b

aH

ea

t fl

ow

(W

/g)

Temperature (oC)

Fig. 2.13 DSC curves of (a) virgin fiber and (b) etched fiber

Table 2.2 Melting temperature, heat of fusion and degree of crystallinity

of the virgin and etched fibers

Fiber Melting temperature,

Tm (°C)

Heat of fusion,

∆Hf (J/g)

Degree of crystallinity,

χc (%)

Virgin 219.4 64.5 28.0

Etched 215.8 57.8 25.1

2.3.3 PANI coated Nylon fiber

2.3.3.1 Strength of the fiber

The effect of etching time on the strength of PANI coated fiber was evaluated by

etching the fiber to different time durations at 60-65 °C with 0.012 N chromic acid,

and subsequently effecting in situ polymerization as explained in section 2.2.4. The

breaking load of the PANI coated fibers was plotted against the time of etching as

shown in fig. 2.14. Generally, PANI is known as a very rigid material, and the tensile

strength of most PANI blends or composites has a decreasing tendency as the

concentration of PANI increase [36, 37]. This trend is seen in our case also. An

increase in etching time leads to better surface etching, enhancing the physical

adsorption of aniline molecules on the fiber surface. This result in the formation of


61

more amount of PANI on the fiber surface leading to a better coating of PANI layer

strongly bonded to the surface. There is a sudden fall in the strength of the fiber

during the first hour, after which there is a gradual decrease and it stabilizes at an

etching time of 4 h. Beyond 4 h, again, the strength shows a drastic decrease.

Comprehending fig. 2.7 and fig. 2.14, it can be inferred that the PANI coating does

not affect the fiber strength significantly up to an etching time of 4 h, beyond which

there is a rapid fall.

0 1 2 3 4 5 6

0.040

0.045

0.050

0.055

0.060

0.065

0.070

Bre

akin

g lo

ad

(kN

)

Time of Etching (h)

Fig. 2.14 Variation of breaking load of PANI coated fiber with etching time

2.3.3.2 DC electrical conductivity

Fig. 2.15 presents the variation of conductivity of PANI coated fibers with time of

etching. The conductivity of the PANI coated virgin Nylon fiber is 4.22 × 10-2

S/cm

(log conductivity of -1.37 S/cm). An etching time of 1 h itself increases the

conductivity to 1.73 × 10-1

S/cm. i.e. A four-times increase in conductivity is

observed for the fibers that have been given etching treatment for 1 h compared to

the unetched ones. The conductivity increases almost steadily to 3.72 × 10-1

S/cm at

4 h. As the etching time is again increased, there is a further increase in conductivity.

Obviously, the conductivity should increase on increasing the etching time as the

amount of PANI formed on the surface increases. Since the strength of the fiber

decreases beyond 4 h, and a reasonable conductivity is obtained at 4 h, the optimum

time of etching is deduced to be 4 h. At an etching time of 4 h, the fiber conductivity

Chapter 2

62

is 3.72 × 10-1

S/cm (log conductivity of -0.42 S/cm), approximately 8 times than that

of the unetched fibers. Genies and co-workers prepared conducting materials made

with polyaniline and glass textiles [2]. The glass textiles were given a chemical

treatment with 20 % sulphuric acid solution for 6 h before giving the PANI coating,

to wash off the glass of any surface agents. They could achieve only a conductivity in

the range of 1.5 × 10-3

to 5.2 × 10-2

S/cm. By plasma treatment, Oh et al. reports only

a fabric log conductivity of –2.2 to –1.6 S/cm [14]. Thus, chemical etching is

effective in giving improved surface adhesion and better coating of PANI, resulting

in improved conductivity than the expensive plasma treatment method.

0 1 2 3 4 5 6

0.0

0.1

0.2

0.3

0.4

0.5

Co

nd

ucti

vit

y (

S/c

m)

Time of etching (h)

Fig. 2.15 Variation of conductivity of the PANI coated fibers with time of etching

2.3.3.3 Scanning electron microscopy

Figs. 2.16(a) and (b) show the micrographs of PANI coated etched fibers and PANI

coated virgin fibers, respectively. PANI particles grown on the fiber surface are

visible. Such globular deposits have been observed in several reports on PANI and

polypyrrole films produced on fibers or micro particles [14, 38-41]. Pud et al. also

reported the morphology of PET/PANI composites with atomic force microscopy

(AFM), and they observed that PET and the undoped form of the PET/PANI

composite exhibited comparatively flat surfaces, whereas doping led to the

emergence of mountainous features [42]. With etched fibers, better deposition of

PANI on the fiber surface is obtained. Thus, etching is effective in increasing the

deposition of PANI.


63

Fig. 2.16 Scanning electron micrographs of (a) PANI coated etched fibers and

(b) PANI coated virgin fibers

2.3.3.4 Infrared spectroscopy

The spectra of PANI coated etched fiber and PANI coated virgin fiber are presented

in figs. 2.17(a) and (b), respectively.

3500 3000 2500 2000 1500 1000

AT

R u

nit

s

Wavenumber (cm-1)

b

a

Fig. 2.17 Infrared spectra of (a) PANI coated etched fiber and

(b) PANI coated virgin fiber

The N-H/C-H intensity ratio reduced from 1.63 to 0.388 for PANI coated etched

fiber, which can be attributed to the shielding of the Nylon fibers as a result of the

PANI coating on the fiber surface. We expect the ratio to be higher for PANI coated

etched fiber since more N-H bonds are formed during etching. On the contrary, the

PANI coated virgin fiber gives a value of 0.802, which is higher than that of the

a b

Chapter 2

64

PANI coated etched fiber. The increase is due to the lesser shielding of the Nylon

fibers as compared to the latter, as there is a lesser amount of PANI coating on the

unetched fibers.

2.3.3.5 X-ray diffraction analysis

The X-ray diffraction patterns of the PANI coated fibers (fig. 2.18) differ from the

uncoated Nylon fibers (fig. 2.11) suggesting that PANI coating affects the crystal

structure of Nylon fiber. In the XRD spectrum of the PANI coated fibers, in addition

to the peaks of the fiber, diffraction peaks of PANI at 2θ ≈ 17.5 °, 27

° and 30

° can be

seen. The intensity of the α1 and α2 peaks of the uncoated Nylon fibers (fig. 2.11)

reduces upon PANI coating (fig. 2.18) due to a decrease in the percentage

crystallinity of the fiber. Oh et al. too have reported such a decrease in the intensity

of the peaks after PANI coating [13]. The α1 and α2 peaks can be seen at 2θ ≈ 20.1 °

and 23.6 ° for the PANI coated etched fiber. These are seen at 2θ ≈ 20.5

° and 24.1

°

for the PANI coated virgin fiber. The α2 values slightly shifted to higher side after

PANI coating, in comparison to the virgin and etched fibers.

10 20 30 40

0

2

4

6

8

10

12

14

Inte

nsit

y c

ou

nts

Two-theta-scale

a b

Fig. 2.18 X-ray diffraction patterns of (a) PANI coated etched and


2.3.3.6 Thermal stability

The thermal degradation characteristics of PANI and PANI coated fibers as extracted

from TGA data is given in table 2.3. The TG curves are presented in fig. 2.19. The


65

PANI coated fibers exhibit a lower onset decomposition temperature than the virgin

fiber, but the slope of the decomposition curve is more gentle [25]. The peak

degradation temperatures of the etched and PANI coated etched fibers are 453.4 °C

and 452.5 °C, respectively, and that of the virgin fiber and PANI coated virgin fiber

are 460.9 °C and 460.5

°C, respectively (tables 2.1 and 2.3). This indicates that the

PANI deposition does not affect the thermal stability of the fiber. The onset

temperature, weight loss during degradation, weight remaining at peak degradation

temperature and the temperature at 50 % weight loss of the PANI coated, etched and

virgin fibers does not show variation. Even though etching causes a reduction in the

thermal stability of the fibers, PANI coated fibers does not show much variation in

the thermal characteristics. The thermal characteristics of the PANI coated fibers do

not show the significant improvement in thermal stability that has been reported for

other conductive polymer composite systems [43, 44]. Similar results in the case of

PANI/Nylon composite systems have been reported elsewhere [32]. Abraham and

coworkers reported poor thermal stability of PANI/Nylon composite films when

compared to pristine Nylon film [45]. PANI coated etched fibers leave slightly more

residue than the PANI coated virgin fibers showing that PANI deposition is higher

on the former.

Table 2.3 Thermal decomposition data of the PANI coated fibers

Fiber PANI coated etched PANI coated virgin

Onset temperature (°C) 293.5 291.5

Peak degradation temperature (°C) 452.9 460.0

Weight loss (%) 92.3 93.2

Weight remaining

at Peak degradation temperature (%) 33.4 33.9

Temperature at 50 % weight loss (°C) 444.8 452.2

Residue (%) 2.57 2.43

Chapter 2

66

0 100 200 300 400 500 600 700 800 900

0

20

40

60

80

100

Weig

ht

(%)

Temperature (oC)

a b

Fig. 2.19 TG curves of (a) PANI coated etched fiber and


Fig. 2.20 presents the DSC curves of the PANI coated fibers.

180 200 220 240

b

a

Heat

flo

w (

W/g

)

Temperature (oC)

Fig. 2.20 DSC curves of (a) PANI coated etched fiber and


The melting temperature, heat of fusion and degree of crystallinity are presented in

table 2.4. PANI coating on the Nylon fiber reduces the degree of crystallinity and

heat of fusion. The melting temperature also shifts to a slightly lower value. This can

be visualized in the DCS curves too. This means that the crystalline regions are

partly destroyed by the formation of PANI on the fiber surface. Similar observations

have been reported by Oh et al. [13, 14] and Byun et al. [25]. Diffusion of aniline


67

into the Nylon fiber presumably disturbs the crystalline regions and decreases its

thermal properties. The aniline diffused into the Nylon fibers localizes in the

amorphous regions and acts a plasticizer, disturbing the orientation and arrangement

of Nylon macromolecular segments [46].

Table 2.4 Melting temperature, heat of fusion and degree of crystallinity

of PANI coated fibers

Fiber

Melting

temperature,

Tm (°C)

Heat of

fusion,

∆Hf (J/g)

Degree of

crystallinity,

χc (%)

PANI coated

etched 218.0 61.5 26.7

PANI coated virgin 211.8 58.7 25.5

2.4 Conclusions

Electrically conducting fibers were prepared by in situ polymerization of aniline on

Nylon fibers. Chemical etching of Nylon fibers using chromic acid prior to in situ

polymerization is found to be effective in improving the adhesion of PANI to the

fiber surface and conductivity. The etching process involves hydrolysis of the amide

linkages. The crystallinity and thermal stability of the fiber reduces upon etching.

The etching process roughens the fiber surface and slightly reduces the strength of

the fiber. PANI deposition on the etched fibers does not further lower its strength and

thermal stability. Better deposition of PANI on the fiber and improved conductivity

is attained by the etching treatment of the fibers. By giving an etching treatment for

4 h, 8 times increase in conductivity is attained. The crystallinity of the fiber reduces

on PANI coating.

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Chapter 2

70

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